1. Field of the Invention
The present invention relates to a method of manufacturing a color cathode ray tube and more particularly, it relates to method of degaussing the color cathode ray tube of the shadow mask type in a manufacturing step of the color cathode ray tube.
2. Description of the Related Art
The color cathode ray tube of the shadow mask type includes a panel section provided with a substantially rectangular face plate and a skirt projected backward from the face plate, a funnel section connected to the panel section, and a neck section continuous from the funnel section. These three sections maintain the interior of the color cathode ray tube as airtight closed and under vacuum. An electron gun assembly for generating electron beams is housed in the neck. A deflection yoke for generating magnetic field is located on the funnel and neck. A degaussing coil for degaussing magnetized parts or components of the tube is also located on the funnel. A phosphor screen is formed on the inner face of the face plate at the panel. A substantially rectangular shadow mask is arranged in the tube, facing the phosphor screen with a certain interval interposed relative to the face plate. The shadow mask is made by a thin metal plate and has a plurality of slits. A mask frame is located around the shadow mask. Plural mask supports, elastically deformable, are welded to the mask frame. Plural stud pins which are engaged with the mask supports are arranged on the inner face of the skirt at the panel section. An inner magnetic shield is attached to the mask frame on the side of the neck to prevent a magnetic field outside, such as earth magnetism, from acting on the electron beams emitted from the electron gun assembly.
In the case of the color cathode ray tube of the shadow mask type, the three electron beams emitted from the electron gun assembly are deflected in horizontal and vertical directions by magnetic field generated by the deflection yoke and then are converged upon the slits of the shadow mask. The electron beams converged upon the slits of the shadow mask land onto the phosphor screen of the face plate at the panel section. The phosphor screen has three kinds of fluorescent stripes which are alternately arranged to form the phosphor screen. When these fluorescent stripes are shot by the three electron beams passing through the slits of the shadow mask, they emit lights of three colors red, green and blue. In short, the slits of the shadow mask serve to direct the three electron beams to their corresponding fluorescent stripes which emit the lights rays of red, green and blue colors.
In the case of the color cathode ray tube, its shadow mask, mask frame, inner magnetic shield and the like are usually made of magnetic material such as low carbon steel. When they are magnetized by outside magnetism such as earth magnetism, therefore, their remnant magnetism shifts paths of the electron beams. When the paths of the electron beams are shifted in this manner, the electron beams cannot be landed correctly onto the phosphor screen. The color purity in the color cathode ray tube is thus reduced. This makes it necessary to demagnetize their remnant magnetism to prevent the color purity from being reduced.
Conventionally, magnetized parts or components of the color cathode ray tube are demagnetized by the degaussing coil in the following three cases.
1) When the property of a color cathode ray tube completed in the course of manufacturing color cathode ray tubes is tested.
2) When the performance of a completed television set in which the color cathode ray tube and other components have been incorporated in the course of manufacturing television sets is tested.
3) Whenever the television set which is in normal use is switched on.
In the case of testing the property of the color cathode ray tube, the color purity is measured as a point to evaluate the quality of image. This color purity is measured after remnant magnetism is degaussed according to the method which will be described later. In another case of incorporating the color cathode ray tube and other components to form the completed television set, remnant magnetism is demagnetized to color-adjust the color cathode ray tube.
U.S. Pat. No. 4,737,881 uses a resonance circuit to degauss the remnant magnetism of magnetic components. Frequency (f) of degaussing current flowing through the resonance circuit is denoted by f=1/2.pi..sqroot.LC wherein L represents inductance and C capacitance. The inductance of a degaussing coil used in this resonance circuit is several mH (millihenries) and the capacitance of a condenser is several uF (microfarads). The frequency of degaussing current flowing through the resonance circuit becomes several ten kHz, accordingly. Energy (E) of degaussing magnetic field is denoted by E.varies.2 .pi..sqroot.LC=1/f and energy (E) is therefore inversely proportional to frequency. In other words, degaussing energy (E) becomes smaller and smaller as frequency (f) of degaussing current becomes higher and higher. When frequency (f) becomes high, therefore, magnetized components cannot be sufficiently degaussed by the degaussing means.
There is a well known resonance circuit which can oscillate at a frequency lower than 100 Hz. This resonance circuit needs a capacitor having a capacitance (C) of several F (farads) and a coil having inductance (L) of several H (Henries) to oscillate at the frequency lower than 100 Hz. This causes the resonance circuit to become larger in size than the common one used for the color cathode ray tube and its cost to become ten times that of the common one. Therefore, a resonance circuit is not practical.
When a resonance circuit which can oscillate at the frequency of several tens of kHz is used to demagnetize the remnant magnetism of parts of the color cathode ray tube, however, its degaussing energy is small because the frequency of its degaussing current is quite high. The magnetized parts cannot be sufficiently demagnetized by this resonance circuit, accordingly. When the frequency of a degaussing magnetic field is compared with that of a vertically deflecting magnetic field generated by the deflection yoke, the latter is lower than the former. When degaussing and vertically deflecting magnetic fields are applied to the magnetized components of the tube at the same time, therefore it becomes more difficult for the magnetized components to be demagnetized because energy of vertically deflecting magnetic field is higher than that of degaussing magnetic field. This makes it necessary to use a delay circuit intended to flow deflecting current after the magnetized components are demagnetized.
Moreover, magnetized parts of the color cathode ray tube are demagnetized by the degaussing coil supplied with the current whose frequency is the same as the commercial frequency. The frequency of the vertical deflection current is the same as the commercial frequency in most regions in the world. FIG. 1 shows how degaussing magnetic field 2 generated by the transient, attenuating A.C. current and deflection magnetic field 4 generated by the vertical deflection current change with time. As is shown in FIG. 1, degaussing magnetic field 2 and deflection magnetic field 4 have the same frequency. Degaussing magnetic field 2 is shown as being in phase with deflection magnetic field 4, but normally, degaussing magnetic field 2 is not generated in synchronism with deflection magnetic field 4. Therefore, when degaussing magnetic field 2 is applied for the degaussing of the color cathode ray tube, a phase shift is likely to occur between degaussing magnetic field 2 and deflection magnetic field 4. FIG. 2 shows magnetism flux density distribution 6 relating to the vertical deflection magnetic field generated when the deflection coil is supplied with a vertical deflection current. In FIG. 2, magnetic flux densities are plotted against the ordinate and distances measured from the neck are plotted against the abscissa. As is indicated by magnetism flux density distribution curve 6, the magnetic flux density is as high as 5 gauss even at position 8 where the end of the magnetic shield is located. Since, therefore, the degaussing magnetic field and the vertical deflection magnetic field are superimposed on each other in the location of the magnetic shield, a hysteresis loop of a magnetic member applied with both degaussing magnetic field 2 and deflection magnetic field 4 is not symmetric with reference to the origin, and the magnetic shield retains the magnetism arising from the above-mentioned phase shift, even after it is degaussed. FIG. 3 shows hysteresis loop 10 of the magnetic field. As is shown, hysteresis loop 10 of a magnetic member applied with both degaussing magnetic field 2 and deflection magnetic field 4 is transformed or shifted from normal hysteresis loop 12, which is indicated by the broken lines with the reference numeral of "12". Although, in FIG. 3, hysteresis loop 12 is rotation-symmetric with reference to the origin, hysteresis loop 10 is not. FIG. 4 shows a detailed hysteresis curve obtained when a magnetic material is degaussed by applying a degaussing magnetic field thereto. FIG. 5 shows a degaussing magnetic field generated when a degaussing current flows through the degaussing coil, and also shows a vertical deflection magnetic field generated when a vertical deflection current flows through the deflection yoke. The magnetic flux density and magnetic field strength at time a in FIG. 5 are indicated at point a in FIG. 4, and those at time c in FIG. 5 are indicated at point c in FIG. 4. Likewise, times b and d-h in FIG. 5 correspond to points b and d-h, respectively.
Otherwise, the magnetic flux density and the magnetic field strength at time a in FIG. 5 are indicated at point a' in FIG. 4 when the deflection yoke does not generate a vertical deflection magnetic field. Those at time c in FIG. 5 are indicated at point c' in FIG. 4 when the yoke does not generate the magnetic field. Likewise, times e,g in FIG. 5 correspond to points e', g', and times b, d, f and h in FIG. 5 correspond to proximate points (not shown) of points b, d, f and h respectively when the yoke does not generate the magnetic field. As a result, the hysteresis curve in FIG. 4 is shifted from a hysteresis curve (not shown) in the case of the magnetic shield applied with only degaussing magnetic field. Shift distance between point c and point c' is shorter than shift distance between point a and point a'.
Therefore, when magnetic shield is applied with both degaussing magnetic field 21 and vertical deflection magnetic field 22, its hysteresis curve 20 can be represented in the manner indicated in FIG. 4. Accordingly, magnetic field strength Hb at time b is greater than magnetic field strength Hd at time d since magnetic shield is applied with vertical deflection magnetic field 22. The decrease in quantity .increment.Hd of magnetic field strength Hd is greater than the decrease quantity .DELTA.Hb of magnetic field strength Hb. Therefore, hysteresis curve 20 is formed asymmetrically shown in FIG. 4. Moreover, the asymmetrical hysteresis curve is shifted in one direction during this degaussing since vertical deflection magnetic field 22 at points a, b, c and d in FIG. 5 are equal to magnetic field 22 at points e, f, g and h respectively. As a result of this degaussing, the hysteresis curve converges at point r in FIG. 4, and the remnant magnetism at converging point r in Br. In short, the remnant magnetism does not decrease to 0. Since, converging point r moves due to the phase difference between the degaussing magnetic field and the vertical deflection magnetic field, remnant magnetism Br varies accordingly.
FIG. 6 shows how the landing point of an electron beam is shifted from its initial landing point on the phosphor screen corner, wherein the initial landing point is obtained when the color cathode ray tube is degaussed at the first time by using the above-mentioned degaussing method, and the other landing point is obtained when the color cathode ray tube is also degaussed at the other time. In FIG. 6, the ordinate represents the distance between the initial landing point and the other landing point, while the abscissa represents how many time the degaussing method has been used. The curve shown by a solid line interrupted by two dashed lines represents the right bottom corner of the screen. The curve shown by a dashed line represents the left bottom corner of the screen. The curve shown by a solid line represents the upper right corner of the screen. The curve shown by a solid line interrupted by a single dashed line represents the upper left corner of the screen. As can be understood from FIG. 6, the maximum shift distance is 33 .mu.m and the average shift distance is 11 .mu.m. Since, as noted above, the remnant magnetism varies in accordance with the phase difference between the degaussing magnetic field and the vertical deflection magnetic field, therefore the distance through which the landing point is shifted varies in accordance with the phase difference.